My two year old utterly flipped out when we dropped off my oldest at cub scouts. (Child 2 and 4 were at home with Dad.) Ten minutes later he was still yelling about his brother. I spent most of the time trying to calm him down with light conversation, but nothing worked until I asked him, "are you worried about him or do you miss him?"
"Is he your best brother?"
"No, my best friend."
Awe, that was sweet, that declaration was what he needed to calm down.